Incinerator for burning waste and a method of utilizing same

ABSTRACT

An incinerator for burning waste including a hollow body having an opened upper end and an opened lower end, a bottom plate closing the lower end, a central opening provided in the bottom plate for introducing pressurized oxidizing gas into the hollow body, a plurality of circumferentially disposed openings provided in the bottom plate for introducing pressurized oxidizing gas into the hollow body, an oxidizing gas feeding means for supplying gas to the central opening and the circumferentially disposed openings, heat transfer medium particles provided in the hollow body and fluidized by the oxidizing gas introduced into the hollow body and a means for feeding waste into the hollow body.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an incinerator for burning industrialwaste such as dust or sludge discharged from chemical plants and fromthermal power plants, and waste such as municipal refuse.

2. Prior Art

Exhaust gas discharged from the boilers which burn fossil fuel containsdust that is collected by a bag-filter, cyclone collector or electricprecipitator. However, recently, machines and apparatuses for use insteel industry, chemical industry and electric power industry havebecome very large, thereby increasing the amount of dust collected perdust collector to a very high extent. For example, the amount of dustcollected by dust correctors from exhausted gas from a boiler for thegeneration of electric power of 1 million Kw output is about 1-3 m³ perhour, though the amount depends on the amount of dust contained in theexhaust gas, the collecting efficiency of the dust collector used andthe bulk density of the dust.

Dust contained in the conbustion gas from the burning of a fossil fuelsuch as heavy oil has the following characteristics.

(1) The bulk specific gravity is as small as 0.1-0.2 g/cc. (2) Theamount of combustible components (carbon and sulphuric acid) is about90%.

(3) The amount of incombustible components (ash) is about 10%, butcomposed mostly of elements such as vanadium (V) and nickel (Ni).

The dust described above has a large bulk specific gravity, difficult tohandle and transport efficiently, and moreover, creates a publicnuisance by scattering dust and acidifying the soil. Therefore, it isdifficult to dispose of this type of dust. Consequently, a method hasbeen practised that the dust described above is incinerated to decreaseit in volume and elements remaining are concentrated to be recovered.

On the other hand, sludge discharged from thermal power plants isproduced during processing of waste water from ejectors forairflow-transporting of dust collected by dust collectors, waste waterfrom coal yards, and the like. The disposal of the sludge describedabove also causes a public nuisance. Therefore, a method has beenpractised that sludge after being dehydrated is incinerated in the samemanner as the dust.

Heretofore, rotary kilns and fluidized bed furnaces have been mainlyused as an incenerator for wastes such as dust and sludge. According tothe method using a rotary kiln, an additive functioning as a meltingpoint raiser and a granulation promoter, such as magnesium hydroxide, isapplied to the dust or sludge which is then formed into pellets througha pelletizer and thrown into a rotary kiln so as to be incinerated orcalcined. However, according to this method, sludge is not fullyincinerated. To better incinerate it, it may be required that thedimensions of the rotary kiln should be enlarged and additional devicesshould be provided for treating harmful gaseous components and malodor.Further, low melting point compounds contained in the waste such asvanadium pentoxide and sodium chloride stick to the inner walls of thekiln. Therefore, attention should be paid to the distribution oftemperature in the kiln. On the other hand, according to the methodusing a fluidized bed furnace, a heat transfer medium such as sand isfluidized by ejecting gas into the furnace to form a fluidized bed inwhich waste is incinerated. This method has such an advantage that, ingeneral, powdered materials such as dust and sludge are suitable forfluidization and are incinerated easily, and the heat capacity of thefurnace is high, so that self-combustion can be readily sustained;whereas, there is such a disadvantage that incombustible componentswhich can be molten, particularly vanadium or sodium compounds aremolten in the furnace and stick to the inner wall of the furnace. Forexample, the dust collected from combustion gases of petroleum fuelscontains about 10% ash (incombustible components), which is composedmainly of compounds of vanadium, particularly vanadium pentoxide (V₂ O₅)whose melting point is 670° C. Consequently, when the dust isincinerated in an ordinary fluidizing bed furnace in the self-combustiontemperature region ranging from 700° C. to 800° C., vanadium pentoxide(V₂ O₅) is molten and exudes onto the surface of dust particle, wherebythe dust particle sticks to the inner wall and the inlet for the gas tothe furnace, for example, a perforated plate for introducing gas intothe furnace. As a result clinkers are formed in the fluidized bed sothat fluidizing of the waste is interrupted and the combustion in afluidized state is made difficult.

Furthermore, when ammonium (NH₃) gas is used as an electrolyte foraccelerating deposition of dust in the electric precipitator or as areducing agent for eliminating nitrogen oxides (NO_(x)) in combustiongas of petroleum fuels, the dust collected from the combustion gascontains a large amount of ammonium sulfate [(NH₄)₂ SO₄ ]produced in thereaction of NH₃ with SO₃ from the sulfur components contained in thefuel. The ammonium sulfate becomes molten in the furnace in the samemanner as the vanadium compounds, sticks to the inner wall of a furnaceand the like, thus causing troubles similarly to the above.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean incinerator for burning industrial waste such as dust and sludge in afluidized bed at a high combustion efficiency.

It is another object of the present invention to provide an incineratorfor burning industrial waste, wherein the sticking of dust to the innerwall thereof due to the melting of low melting point compounds can beeliminated.

It is a further object of the present invention to provide anincinerator for burning industrial waste wherein valuable metals ormetallic compounds contained in the waste can be recovered as ash aftercombustion.

It is a still further object of the present invention to provide anincinerator for burning industrial waste, wherein waste such ascombustible dust or sludge containing inorganic compounds having lowmelting points is subjected to self-combustion at a relatively lowtemperature.

It is a yet further object of the present invention to provide a processfor burning industrial waste in a fluidized bed at high combustionefficiency.

In accordance with the present invention, there is provided anincinerator, which generally includes a bottom plate provided at thecenter thereof with an upwardly directed opening for introducingpressurized oxidizing gas and having a multiplicity of openings forintroducing pressurized oxidizing gas, disposed in the circumferentialdirections respectively at the inner periphery of the plate, anoxidizing gas feeding means for ejecting the gas from the opening at thecenter of the plate and from the openings at the inner periphery of theplate, respectively, a cylindrical body vertically connected at thelower end thereof to the bottom plate and provided at the upper endthereof with an exit for the discharge of combustion gas, heat transfermedium particles contained in the cylindrical body and fluidized by thegas ejected through the openings in the bottom plate and means offeeding industrial waste into the cylindrical body.

In accordance with the present invention, there is also generallyprovided a process for burning industrial waste, which includes thesteps of forming a swirling blast fluidized bed above a perforated platein an incinerator by blowing oxidizing gas through the perforated platein an upwardly vertical and horizontally circumferential directions tomix the waste in the fluidized bed with a heating medium and burn at thetemperature of 500° C.-1000° C., retaining the resultant combustion gasin the free space above the fluidized bed for a sufficient period oftime required for the combustion of unburned components contained in thecombustion gas, and thereafter discharging the combustion gas from theoutlet above a free space of the incinerator, blowing cooling air intothe outlet for combustion gas so that the temperature of the gasdischarged through the outlet can be maintained within the range from350° C. to 670° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be fully understood from the followingdescription, taken in connection with the accompanying drawings whereinlike elements are given like reference numerals and in which:

FIG. 1 is an elevational section view of a preferred embodiment of anincinerator containing heating medium and waste in accordance with theteachings of the present invention;

FIG. 2 is an enlarged view of part of the incinerator shown in FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 of FIG. 2; FIG. 4is an elevational section view of another preferred embodiment of anincinerator according to the present invention, which is provided with awall-cooling device on the inner wall thereof;

FIG. 5 is a sectional view taken along the line 5--5 of FIG. 4;

FIG. 6 is an illustrative view of a preferred embodiment of atemperature controll system for combustion gas passing through theoutlet of an incinerator according to the present invention;

FIG. 7 is a diagram for illustrating the temperature and pressuredistribution of the combustion gas in an incinerator according to thepresent invention as compared with those of a conventional incineratorusing a fluidized bed;

FIG. 8 is a diagram for illustrating preferable temperature ranges inthe incinerator and the way of thermal decomposition of the materials inthe incinerator within those temperature ranges according to the presentinvention;

FIG. 9 is a diagram for illustrating the relationship betweenresidential time of combustion gas in the incinerator and concentrationof ammonium gas at the outlet of the incinerator;

FIG. 10 is a flow of a preferred embodiment of the process for burningindustrial waste by use of an incinerator according to the presentinvention; and

FIG. 11 is a flow diagram of another preferred embodiment of the processfor burning industrial waste by use of the incinerator according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

A general outline of an incinerator using the fluidized bed according tothe present invention is shown in FIG. 1, and the details of thefluidized bed in FIG. 2. In the drawings, the cylindrical body 12 of theincinerator 10 is of a tower-like shape and on the outer surface ofwhich is applied a thermally insulating material. The body 12 isprovided at the bottom thereof with a cone-shaped perforate plate 14having a multiplicity of nozzles 16 for feeding pressurized air whichgives intensive fluidized swirls to a fluidized heat transfer medium 32contained in the body 12 and which is used as oxidizing gas. As shown inFIG. 3, nozzles 16 are disposed in circumferential directions of thecone-shaped plate, respectively. A pressurized air chamber 18 isprovided beneath the plate 14. The pressurized air chamber 18 is fedwith pressurized air from a pipe 20. Additionally, the cone-shapedperforate plate is provided at the lower end thereof with a pressurizedair feed pipe 24 whose lower end is a combustion residue discharge pipe25. An air feed nozzle 19 is inserted through the central portion of thepressurized air feed pipe 24 and pressurized air is blown into theincinerator through the air feed nozzle 19. Surrounding the pressurizedair feed pipe 24 is a pressurized air chamber 26. Air is fed throughnozzles 27 formed in the pipe wall of the pressurized air feed pipe 24so that the pipe portion can be prevented from being blocked bycombustion residue.

Waste material is fed into the incinerator through the side walls at thelower portion of the incinerator 10 by waste feeding means (screwfeeder) 28. Additionally, a burner 30 for the purpose of starting orassisting burning is provided through the side wall of the incinerator.Heat transfer medium particles 32, such as sand, is contained in theincinerator. Heat transfer medium 32 is fluidized in the arrow-markeddirection by air blown out of nozzles 16 and pipe 19, intensely swirledto be formed into a fluidized bed, heated to a high temperature andmixed with the waste fed by the waste feeding means 28. As a result anycombustible components of the waste is burned.

The temperature of the fluidized bed 34 is detected by a temperaturedetector 36. Combustion gases pass through a free space 38, i.e. a spaceupwardly of the fluidized bed 34 in the incinerator, an outlet 40, aduct 42 and discharged to the outside of furnace. The outlet 40 isprovided with a temperature detector 37 which detects the temperature ofexhaust gas. The nozzles 16 formed in the perforate plate 14 arehorizontally disposed in the circumferential direction of the perforatedplate 14. However, the nozzles may be disposed with each having a slightupward inclination to obtain upward swirling.

According to the present invention, when the waste is dust which ismainly composed of ammonium sulfate is burned, the height of the freespace 38 may preferably be 5 m or more in general. Additionally, it ispreferable that the residence time of combustion gases in the free space38 be from about 3.5 to 4.0 sec or more under a temperature of about650° C.-700° C., and that the average flow rate of the combustion gasesbe from 0.5 to 1.0 m/sec.

The temperature of the fluidized bed according to the present inventionis selected from 500° C. to 1000° C. depending upon the properties ofthe waste to be burnt. When general industrial waste is burnt, thetemperature is maintained in the range from 550° C. to 800° C. When dustcorrected from waste gas from boilers for thermal power plant is burnt,the the temperature is maintained in the range from 550° C. to 780° C.and preferrably from 580° C. to 730° C. If the temperature is is lowerthan 550° C., then unburned carbon contained in the dust is not fullyburned; if the temperature is higher than 780° C., then low meltingpoint compounds contained in the combustion residue are unsuitably fusedto the inner wall and the perforated bottom plate of an incinerator.

In order to prevent the low melting point compounds from being fused tothe inner wall in the free space 38 of the incinerator, it is desirableto flow cooling air along the inner side wall and form an air curtainaround the combustion zone in the free space. As a results thetemperature of the inner wall of the incinerator is lower than meltingpoints of the low melting point compounds (in the case of vanadiumpentoxide, 670° C.). An example of the inner wall cooling means isindicated in FIGS. 4 and 5. In the drawings, inserted into theincinerator along the inner wall surrounding the free space 38 are aplurality of air introducing pipes 44 whose openings to the free spaceare formed into upwardly directed slits 46 so that air introducedthrough the air introducing pipes 44 can be allowed to flow upwardsalong the inner wall of incinerator. Air introduced into the incineratorthrough the slits 46 flows along the inner wall of the incinerator inthe direction indicated by an arrow 48 so as to form an air curtain.Thus, the rise in temperature of the inner wall of the free space 38 andthe sticking of dust to the said inner wall due to the melting of thelow melting point compounds can be prevented.

Combustion gases passing through the free space 38 and discharged fromthe outlet above said free space 38 contain therein molten compounds ofvanadium, for example. Hence, if the combustion gases are directlyintroduced to a duct 42, through the outlet 40, the compounds ofvanadium stick to the inner wall surface of the duct 42. For thisreason, nozzles 54 for introducing cooling air through an airintroducing pipe 50 and a ring header 52 may be provided in the innerwall in front of the duct in the upper portion of the free space 38. Thecombustion gases are cooled to lower than the melting points of lowmelting point compounds (e.g. 670° C. in the case of vanadium pentoxide)by sending cooling air into the upper portion of the free space throughthe nozzles. As a result, the low melting point compounds arecoagulated, thereby preventing the compounds from being stuck to theinner wall of the duct 42.

In order to coagulate the low melting point compounds contained in thecombustion gas being discharged from the outlet 40 and reduce thecorrosion by sulphuric acid gas, for example, contained in the exhaustgases, it is possible to provide exhaust gas cooling means andtemperature controlling means therefor in the duct 42 communicated withthe outlet 40. An example of such a means is shown in FIG. 6.

The cooling air feeding means is basically identical with that shown inFIG. 5. In particular, a plurality of cooling air feeding nozzles 56 arearranged in the side wall of the duct 42 adjacent to the outlet 40 forexhaust gases and cooling air is fed to the nozzles 56 through a ringheader 58. The cooling air may be fed by an independent fan or may befed in such a manner that a pipe line 64 is branched from a main pipeline 62 of a main forced fan 60 and the pipe line 64 is connected tosaid ring header 58 through a cooling air flow rate control valve 66.The temperature of exhaust gas flowing out of the outlet 40 is detectedby a temperature detector 68, and the resultant temperature signal issent to a temperature controller 70. The temperature controller 70 sendsout a signal corresponding to the difference from a preset temperaturevalue to a cooling air flow rate control valve 66 provided at thebranched pipe line 64 and controls the flow rate of the cooling air fed.In the case where the dust is collected from waste gas from boilersusing fossil fuels, the setting value for the temperature of thetemperature controller 70 is higher than the dew point of sulphuric acid(about 350° C.) and lower than the melting point of vanadium pentoxide(670° C.), preferably about 500° C. At about 500° C., unburned carbon inthe combustion residue particles accompanied by the combustion gases isstill being burned as embers. However, the low melting point compoundson the outer surfaces of the particles have begun to become solid, andhence, even if the particles are piled up, they do not fuse together.

With the incinerator according to the present invention, the fluidizedheat transfer medium above the perforated plate is fluidized by flowingoxidizing gas through the nozzle 16 and pipe 24 in the vertical andcircumferential directions to make swirls in the fluidized bed and ismixed with the dust in many directions, and hence, the fluidized heattransfer medium is the bed has no static portion at all. Additionally,the heat capacity of the incinerator is large due to the presence of thefluidized heat transfer medium therein, whereby the combustionefficiency becomes high which results in self-combustion of the waste ata relatively low temperature. Furthermore, the cohesion or agglomelationof the waste particles due to the melting of the low melting pointcompounds, the sticking of the waste to the perforated plate and theinner wall of incinerator and the piling of the waste can be preventedby the combustion under intensive stirring and at relatively lowtemperature.

FIG. 7 indicates examples of the characteristics of the conventionalfluidized bed furnace of a flat tray shape having a fluidized bed of adiameter of 600 mm and the incinerator according to the presentinvention having the same diameter as above (the amount of dustprocessing is each 100 Kg/hr). In the drawing, the heat capacities ofthe dust A and B are 1530 Kcal/kg and 2500 Kcal/kg, respectively, andthe height Lc of fluidized bed is 400 mm. From the drawing, thefollowing is apparent.

(1) The distribution of temperature in the longitudinal direction of theincinerator:

in the conventional fluidized bed furnace, both dust A and B suppliedhave a rise in temperature of about 720° C.+20° C. at the bedtemperature at portions 1,000-1,700 mm upwardly of the fluidized bed.This is because the incompletely burned components in the fluidized bedare being burned in the free space, which indicates that the incineratoraccording to the present invention has a higher combustion rate than theconventional fluidized bed furnace when an equal amount (100 kg/hr) isprocessed in each furnace.

(2) The distribution of temperature in the cross-sectional direction ofthe bed:

with respect to the distribution of temperature at points at a pitch of50 mm from the side wall of furnace to the center of furnace and at theheights of 100, 200 and 400 mm from the flange of perforated plate, thefluidized bed according to the present invention has a substantiallyuniform distribution of temperature, whereas the conventional fluidizedbed has a concentrically convex distribution of temperature with thecenter of furnace which is 10°-30° C. higher in temperature than theportions other than the center.

(3) The distribution of pressure in bed:

the back pressures at the points of the above distribution oftemperature were measured. The pressure in the fluidized bed accordingto the present invention is higher by about 50 mm Aq than in theconventional fluidized bed, which indicates, in the fluidized bedaccording to the present invention, the fluidization of the heattransfer medium is active and the pressure distribution across thefurnace has a convex shape due to the effect of the swirl air blast.

(4) The pressure change in furnace:

the pressure change in the conventional fluidized bed is ±25 mmAq-±35mmAq, whereas that in the fluidized bed according to the presentinvention is ±15 mmAq. As compared with the conventional fluidized bed,the combustion rate in the bed is high and the combustion in the freespace is lower than that in the bed, both of which are in accordancewith the results obtained in the above item (1).

As has been described above, it is apparent that the incineratoraccording to the present invention has excellent combustioncharacteristics as compared with the conventional fluidized bed furnace.

FIG. 8 indicates the distribution of temperature in the furnace and thedecomposition process of the components contained in the dust for thecase wherein dust mainly containing ammonium sulfate and carbon isburned by use of the incinerator according to the present invention.

The temperature of the fluidized bed in the incinerator according to thepresent invention is preset to 550° C.-730° C. in consideration of theperfect combustion of carbon and the prevention of the dust frombecoming molten and sticking. This range of temperature is secured bycontrolling the quantity of dust and air supplied to the incinerator.Carbon begins to burn at 450° C. and above in the bed. Although vanadiumpentoxide (V₂ O₅) is molten at 670° C., it never sticks to the furnacewall because the content of vanadium pentoxide is comparatively low andthe dust is intensively stirred in the bed. Ammonium sulfate [(NH₄)₂ SO₄] begins to be softened at about 120° C., liquefied and solidified inseveral minutes. Consequently, it is desirable to use rapid heating,stirring and disintegration during its decomposition and combustion,which suits well to the incinerator according to the present invention.

Ammonium sulfate [(NH₄)₂ SO₄ ] decomposes at two steps in thetemperature range from 288° C. to 490° C., and is finally decomposed toNH₃ and SO₃ as shown in the following formula.

    (NH.sub.4).sub.2 SO.sub.4 →2NH.sub.3 +SO.sub.3 +H.sub.2 O (1)

The NH₃ and SO₃ further reacts in the free space above the bed accordingto the following formulae (2) and (3).

    2NH.sub.3 +3/2O.sub.2 →N.sub.2 +3H.sub.2 O          (2)

    SO.sub.3 →SO.sub.2 +1/2O.sub.2                      (3)

The height of the free space shall be determined so that the residentialtime of the combustion gas required for the completion of the abovereaction (2) is secured.

With respect to the reaction rate constant of NH₃ in the above formula(2), the gas phase oxidation reaction (combustion reaction) rateconstant K of NH₃ is appropriate for this case and is given by thefollowing formula (4).

    K=10.sup.14.61. e(38,700/RT)                               (4)

Additionally, the combustion rate on NH₃ per unit reaction time and perunit reaction volume is given by the following formula (5).

    -d[NH.sub.3 ]=K·[NH.sub.3 ]·[O.sub.2 ]·dt·dV                                 (5)

FIG. 9 is a graphic chart illustrating the relationship between theresidential tiem and the concentration of ammonia in the free spaceportion of the incinerator, both of which are calculated based on theabove formulae (4) and (5) when ammonium sulfate contained in the dustis 30 weight % (c) and 75 weight % (D). In FIG. 9, the result ofmeasuring concentration of ammonium in the combustion gas flowing out ofthe outlet of the incinerator is indicated by black spots. Theconcentration of NH₃ is 14,200 ppm at the inlet of free space portion ofthe incinerator, which is then rapidly burned in the high temperaturezone in the vicinity of the inlet of the free space portion, reduced toabout 500 ppm in one sec., and reduced to less than 1 ppm in theproximity of the furnace outlet in about 3.5-4.0 sec.. Namely, theresidential time required for NH₃ in the gas to be substantially fullydecomposed in the free space portion is about 3.5 sec. and more at about550° C.-700° C., which corresponds to the height of the free spaceportion of about 5 meters and more. Additionally, in this case, theaverage flow rate of the gas in the free space portion was 0.5-1.0m/sec. according to the experimental values.

As described above, NH₃ in the combustion gas is substantially fullyburned, and regeneration and recrystallization of ammonium sulfate[(NH₄)₂ SO₄ ], ammonium sulfite (NH₄ HSO₃) and the like in thecombustion gas after cooling can be prevented, whereas reduction of SO₃according to the formula (2) gives only about 50-85% yield. For thisreason, the temperature of combustion gas is maintained higher than thedew point of SO₃ (about 350° C.) and lower than the melting point of thelow melting point compounds, usually at about 500° C., thus preventingthe duct from being corroded by sulphuric acid and from the sticking ofash.

Reffering to FIG. 10, description will hereunder be given of the processof incinerating dust containing ammonium sulfate and compounds ofvanadium, collected by an electric precipitator in a thermal powerplant, and sludge discharged from the thermal power plant by using theincinerator with the fluidized bed according to the present inventionand using a rotary kiln thereafter. In the drawing, dust collected bythe electric precipitator is contained in a tank 72, and fed to thefluidized bed 80 of an incinerator 76 by a screw feeder 74 for dustfeeding. Dust may be directly supplied to an incinerator by use of thescrew feeder or a pneumatic conveyor without using the tank 72. Whendust collected by an electric precipitator is added to water to beformed into pellets and used as the feed material, an electromagneticvibrating feeder may be used. While, sludge, as another feed material,may be supplied to the incinerator 76 by a screw feeder 78. However, forfeeding a constant quantity of sludge, an extruder with a piston isusable. The incinerator 76 having the fluidized bed 80 according to thepresent invention contains an inactive particles resistant to hightemperature such as sand as the heat transfer medium of said fluidizedbed 80. Part of the pressurized air supplied from forced fan 82 is fedto the fluidized bed 80 through an air chamber 84 and a cone-shapedperforated plate 86. Another part of the air is fed through an airchamber 88, and still other part of it is directly fed into the bed 80from above the perforated plate. The closer to the axis of theincinerator, the denser the perforation of the perforated plate is, i.e.the larger the numerical aperture is, so that the fluidized bed can bereadily formed. As dust and sludge are incinerated, the temperature of afree space 90 is elevated and unburned particles of the sludge and duststick to the inner wall of the free space 76 as a result of low meltingpoint materials such as vanadium oxide and the like. Hence, a layer ofcooling air is formed adjacent the wall surface close to the free space90, with the result that the temperature of the wall surface is cooleddown to less than 670° C. This cooling means includes an air fan 92 forsupplying cooling air, a pipe line 94 connected to the air fan 92, andnozzles 96 for forming the layer of cooling air along the inner wallsurface of the incinerator 76. Additionally, the incinerator 76 isprovided with an auxiliary burner 98 usable for starting the burning ofthe waste and controlling the combustion temperature thereof. A pipeline 100 supplies the air steam, a pipe line 102 supplies a fuel such aslight oil, and a pipe line 104 branched from the pipe line 94 suppliesair for combustion in the auxiliary burner 98.

Exhaust gas containing unburned particles discharged from theincinerator 76 passes through a pipe line 106, and is sent to a singlecyclone 108 where large size particles are separated therefrom. Theexhaust gas further passes through a pipe line 110, a multicyclone 112,a pipe line 114 and an induction fan 116, and is discharged into aboiler flue which constitutes a pipe line to an electric precipitator.At the same time, air is supplied from a pipe line 118 to a pipe line114 for controlling the temperature of the exhaust gas.

All or part of the exhaust gas from multicyclone 112 can be recycled tothe incinerator 76 through line 130 and fan 82 and be used as aoxidizing gas. Thus, the temperature of the fluidized bed 80 and freespace 90 is prevented from rising too high due to the excess amount ofoxygen in the oxidizing gas. The reason why the exhaust gas ispreferably recycled is described below. The amount of an oxidizing gasnecessary to burn the waste in a fluidized bed is not in accorance withthat of the gas required to make fluidized bed. For instance, for thecase where the amount of waste to be burned is small and the airrequired to burn the waste is a small amount, the amount of air requiredto form a fluidized bed becomes too excessive for burning the waste andresults in making the temperature of the bed too high. As a result, theincinerator generates a considerable amount of nitrogen oxides.Generally, 20-50 vol % of the combustion gas is recycled to theincinerator.

The dust containing unburned particles, separated in the single cyclone108 and multicyclone 112 is then supplied to a rotary kiln 124 through apipe line 120 and screw conveyor 122, respectively. The perfectcombustion of the unburned components remaining in the dust and thepelletizing of the dust is performed in the rotary kiln 124, andthereafter, the pellets thus produced are supplied to an ash bunker 126.Exhaust gas discharged from the rotary kiln 124 is sent to the pipe line110 through a pipe line 128.

In the incinerator 76, sludge and dust are diffused over all of thefluidized bed according to the present invention by intense mixing andagitating motions of the fluidized heat transfer medium, andsimultaneously heated and dried by heat of the fluidized heat transfermedium. At this time, the sludge is disintegrated and burned whilefloating moving among the particles of the fluidized heat transfermedium. Volatile combustible components contained in the sludge aregasified and burned, solid combustible components mainly composed ofcarbon are ignited and burned, and the ash thus obtained is atomized. Onthe other hand, ammonium sulfate contained in the dust is instantlydecomposed and gasified; and further the combustion of ammonium and theignition and combustion of carbon contained are performed.

The free space 90 upwardly of the fluidized bed 80 includes a gaseousphase being a continuous phase and a diluted phase of the fluidized heattransfer medium particles acting as a discontinuous phase. Carbonparticles skipping out from the fluidized bed 80 in unburned state arebrought into contact with oxygen and burned, and part of the SO₃generated from the decomposition of ammonium sulfate is dissociated.

In the rotary kiln 124 used as an after combustion furnace, unburnedcomponents remained in the ash are burned in parallel flow and pelletsare produced. In this case, usually a small amount of auxiliary burningis performed and the flow rate of gas is lowered so as to prevent flyash from scattering. The combustion gas discharged from the rotary kiln124 is sent to a multicyclone 112 where ash is collected therefrom, andrecirculated to the electric precipitator in the boiler flue by theinduction fan 116, thus enabling the incineration to act as a perfectlyclosed system.

With this embodiment, sludge and dust are burned mainly in the fluidizedbed, and hence, the load on the rotary kiln which is the aftercombustion furnace becomes less to a considerable extent. As comparedwith the combustion in the conventional rotary kiln, the capacity of thekiln may be made as small as one fifth, for example. Additionally, sincethe heat generation of ash to be burned in the rotary kiln is small,there is not seen any local rise in temperature during combustion,thereby enabling to decrease the amount of ash melting and sticking tothe inner wall of the kiln.

EXAMPLE

In this embodiment, the dust collected from electric precipitator of aboiler for a thermal power plant, is burned by use of the fluidized bedshown in FIG. 1 and according to the flow chart shown in FIG. 11. Thecomposition of the dust includes carbon of 36% by weight, (NH₄)₂ SO₄ of54% by weight, ash of 10% by weight and a trace of water. The calorificvalue of the dust is 3000 Kcal/kg. The conditions of the fluidized bedis as follows:

    ______________________________________                                        Open area of perforated plate                                                                       1.0%                                                    Thickness of perforated plate                                                                       10mm                                                    Diameter of the openings of                                                   perforated plate      3mm                                                     Inner diameter of incinerator                                                                       600mm                                                   Height of the free space                                                                            about 5m                                                Height of the fluidized bed                                                   (at the time of fluidizing)                                                                         400mm                                                   Air supplied to the fluidized bed                                                                   600Nm.sup.3 /hr                                         Temperature of the fluidized bed                                                                    about 730° C.                                    Air supplied as cooling air for                                               cooling the inner wall of the                                                                       190Nm.sup.3 /hr                                         free space portion                                                            Air supplied as cooling air for                                               the outlet of incinerator                                                                           450Nm.sup.3 /hr                                         Temperature of gas at the                                                     outlet of incinerator 500° C.                                          ______________________________________                                    

The exhaust gas discharbed from the outlet of incinerator is supplied tothe byclone 134, sent to an air preheater 136 where the gas is subjectedto heat exchange, supplied to a multicyclone 140, sent to a desulfurizer142 by an induction draft fan 140, and discharged to atmosphere througha stack 144. Part of the cooling air is fed to the free space 148 ofincinerator by a feed draft fan 146, and another part is fed to thefluidized bed 150 through an air preheater 136, respectively.

The amount of ash collected by the cyclone 134, air preheater 136 andmulticyclone 140 are 5.8 kg/hr, 0.5 kg/hr and 3.4 kg/hr, respectively,all of which are supplied to an after combustion furnace (rotary kiln)152 where unburned components are completely burned, and thereafter,discharged to outside. The total amount of ash discharged is 9.4 kg/hrand includes V₂ O₅ of 5.42% by weight, carbon of 0.0% by weight, and(NH₄)₂ SO₄ of 0.0%. The temperature of gases dischared from themulticyclone 138 is 350° C., the amount of gas supplied to thedesulfurizer 142 is 2,850 Nm³ /hr, and the temperature of gas at theoutlet of the desulfurizer 142 is 65° C.

The incinerator according to the present invention has the followingadvantage:

(1) Since self-combustion can be performed at a relatively lowtemperature (less than 780° C.), auxiliary fuel can be minimized ordispensed with and the melting and agglomeration of dust in thefluidized bed, the resultant blocking of the bed and deterioration ofheat resistant materials can be prevented.

(2) Since the burning load per cross-sectional area of the incineratorcan be high, the installation area can be small as compared with therotary kiln furnace, thus enabling one to attain higher burningefficiency.

(3) Since the temperature in the incinerator is low, the amount ofnitrogen oxide (NO_(x)) contained in the combustion gases is low.

(4) Industrial waste such as dust and sludge materials of any shape suchas particles, granules and blocks can be precessed, and there is a largeflexibility with respect to the variations in the amount andcomposition. Additionally, the combustion residue is obtainable inpowdered condition, and hence, easily packed, and valuable metals can bereadily recovered.

It should be apparent that the above discribed embodiments are merelyillustrative of but a few of the many possible embodiments whichrepresent the applications of the principles of the present invention.Numerous and varied other arrangements can be readily devised by thoseskilled in the art without departing from the scope of the presentinvention.

We claim:
 1. A process for burning waste in an incinerator of the type including a hollow body having an open upper end forming an outlet and an open lower end, a bottom plate closing the lower end, a central opening provided in the bottom plate for introducing pressurized oxidizing gas into the hollow body, a plurality of circumferentially disposed openings provided in the bottom plate for introducing pressurized oxidizing gas into the hollow body, an oxidizing gas feeding means for supplying gas to the central opening and circumferentially disposed openings, heat transfer medium particles provided in the hollow body and a means for feeding waste into the hollow body comprising the steps of:forming a swirling blast fluidized bed of heat transfer medium particles above said bottom plate by blowing oxidizing gas through said central and circumferentially disposed openings in upwardly vertical and horizontally circumferential directions; introducing waste into said fluidized bed; burning said waste in said fluidized bed at between 500° C. and 1000° C.; retaining any resulting combustion gas from said burning in a free space above said fluidized bed for a sufficient period of time required for combustion of any unburned components contained in said combustion gas; discharging said combustion gas from said outlet; and blowing cooling air into said outlet to maintain the temperature of the combustion gas discharged through said outlet in a range from 350° C. to 670° C.
 2. A process for burning waste according to claim 1 wherein the temperature of said fluidized bed is within the range of from 580° C. to 730° C. and the temperature of the discharged combustion gas is within the range of 450° C. to 550° C.
 3. A process for burning waste according to claim 1 wherein the retaining time for combustion of any unburned components in said free space is more than 3.5 seconds.
 4. A process for burning waste according to claim 1 further comprising the step of introducing dust collected from combustion gas discharged from said incinerator into a rotary kiln wherein any unburned components contained in said dust are burned.
 5. A process for burning waste according to claim 1 wherein said oxidizing gas is air.
 6. A process for burning waste according to claim 1 further comprises the step of introducing combustion gas recycled from said outlet as at least a portion of said oxidizing gas. 